docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,4 +1,6 @@
 # html-encode
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/Kikobeats/html-encode.svg)](https://greenkeeper.io/)
 ![Last version](https://img.shields.io/github/tag/Kikobeats/html-encode.svg?style=flat-square)
 [![Build Status](https://img.shields.io/travis/Kikobeats/html-encode/master.svg?style=flat-square)](https://travis-ci.org/Kikobeats/html-encode)
 [![Coverage Status](https://img.shields.io/coveralls/Kikobeats/html-encode.svg?style=flat-square)](https://coveralls.io/github/Kikobeats/html-encode)